1. Field of the Invention
The present invention relates to a broadband wireless communication system, and more particularly, to a transmission power report in a broadband wireless communication system.
2. Discussion of the Related Art
As the wireless communication industry is advanced and user requirements for Internet service increase, a related art mobile communication system for providing voice service cannot satisfy user requirements. Due to this reason, wireless communication systems are being recently developed for providing not only voice service but also Internet service.
In such wireless communication systems, as a communication system proposed by Institute of Electrical and Electronics Engineers (IEEE) 802.16, a broadband wireless communication system (called Broadband Wireless Access (BWA)) using Orthogonal Frequency Division Multiplexing Access (OFDMA) has a broader data bandwidth than a related art wireless communication technology for voice service, and thus transmit much data for a relatively short time and enable the efficient use of a channel because all users share the channel.
In a wireless communication system based on IEEE 802.16, a base station determines the transmission power of a corresponding terminal, the size of uplink transmission data that is to be allocated to the terminal, and/or the Modulation and Coding Scheme (MCS) level of the terminal by using the transmission power report provided by the terminal. Specifically, the base station calculates the power headroom of a corresponding terminal by using the maximum transmission power of the terminal included in an SS Basic Capability Request (SBC-REQ) message and a transmission power included in the transmission power report that is transmitted from the terminal, and determines the transmission power of the terminal with the calculated power headroom.
For example, as illustrated in FIG. 1, when the maximum transmission power of a corresponding terminal is 23 dBm and a transmission power included in the transmission power report that is transmitted from the terminal is 13 dBm, the power headroom of the terminal is 10 (i.e., 23-13) dBm. In this case, when data having the same MCS level and same size is allocated to the terminal, a base station increases the transmission power of the terminal by 10 dBm.
Herein, the transmission power report that is transmitted from the terminal to the base station is defined as the power density of a burst including the transmission power report, in which case the power density is changed according to the MCS level (hereinafter referred to as a current MCS level) of the burst including the transmission power report based on the Open Loop Power Control (OLPC) rule of IEEE802.16e. Therefore, when the normalized C/N (which is transferred to all terminals with a broadcasting message) is changed according to the current MCS level, the power headroom of a corresponding terminal is changed according to the current MCS level of the terminal even though wireless environments are the same, and thus, the transmission power of the terminal is also changed according to the current MCS level of the terminal.
As an example, as illustrated in FIG. 2, if a current MCS level is Quadrature Phase Shift Keying (QPSK) ½, it can be seen that a difference of between power headrooms based on each MCS level equal to a different between normalized C/Ns corresponding to each the MCS levels. Alternatively, if the current MCS level is changed from QPSK ½ to 16 Quadrature Amplitude Modulation (QAM) ½, it can be seen that power headroom based on each MCS level is changed as illustrated in FIG. 3.
That is, as seen in FIGS. 2 and 3, although a transmission power reported by a corresponding terminal is constant, when a current MCS level is changed, the power headroom of the terminal is changed.
Accordingly, a base station is required to know a current MCS level in advance, for accurately calculating the power headroom of a terminal. For this end, there are a method where the physical layer (PHY) of the base station transfers both the current MCS level and the transmission power report transmitted from the terminal, to a MAC layer, and a method where the MAC layer accurately memorizes MAP information of a frame that has received the transmission power report. However, it is difficult to apply the methods to both the physical layer and MAC layer.
Considering the time delay between the scheduling time and receiving time of a burst, particularly, the burden given to a base station increases further because the base station is required to memorize the MCS levels of respective bursts for two frames and three frames.